Hitherto, the Unexamined Japanese Patent Publication No. 330314/1992 described a catalytic converter for reducing and decomposing NO.sub.x contained in exhaust gas from a diesel engine. According to that document, a catalytic converter comprises an NO.sub.x catalyst of copper-zeolite, disposed in an exhaust path from a diesel engine; a hydrocarbon sensor, disposed in the exhaust path on the upstream side of the NO.sub.x catalyst, for detecting a concentration of the hydrocarbons in the exhaust gas; and a hydrocarbon fuel injecting means, disposed in the exhaust path upstream of the hydrocarbon sensor, for injecting the fuel into the exhaust path in accordance with the detection by the hydrocarbon sensor in order to keep the concentration of hydrocarbons in the exhaust gas within a predetermined range.
By this construction, a suitable amount of the fuel is injected into the exhaust path by the fuel injecting means in accordance with the detected value of the concentration of hydrocarbons in the exhaust gas so as to keep the concentration of hydrocarbons in the exhaust gas within a predetermined range. The fuel, injected by the fuel injecting means, acts as a reducer of the hydrocarbons and activates the NO.sub.x catalyst of copper-zeolite in cooperation with oxygen contained in the exhaust gas from the diesel engine. As a result, it accelerates the reduction of NO.sub.x in the exhaust gas.
In that document, however, the hydrocarbon sensor is disposed on the upstream side of the NO.sub.x catalyst and the fuel is injected by the fuel injecting means in accordance with the detected value from the hydrocarbon sensor in order to keep the concentration of hydrocarbons within a predetermined range. In this construction, the NO.sub.x in the exhaust gas is properly reduced, but the NO.sub.x catalyst in use deteriorates with the lapse of time to lower its performance of purifying NO.sub.x.
In general, the NO.sub.x purifying performance of an NO.sub.x catalyst is improved by increasing the additive amount of reducer hydrocarbons. Therefore, when the NO.sub.x catalyst deteriorates, it is necessary to correctively increase the additive amount of reducer hydrocarbons, in accordance with the deterioration of the NO.sub.x catalyst, for keeping the exhausted NO.sub.x concentration below a predetermined level. This operation requires an accurate detection of the degradation in the performance of an NO.sub.x catalyst. Although it is desired to develop an optimum apparatus and an optimum method for this purpose, they have not yet been realized.